Hourglass Nebula, MyCn18
Position: R.A. 13h 39m 29.68s
Dec. -67° 22' 38.79"
8,000 light-years (2,500 parsecs)
July 30, 1995
NASA, R. Sahai, J. Trauger (JPL), and the Hubble Heritage team
is an image of MyCn18, a young planetary nebula located about 8,000 light-years
away, taken with the Wide Field and
Planetary Camera 2 (WFPC2) aboard NASA's Hubble Space Telescope (HST).
This Hubble image reveals the true shape of
MyCn18 to be an hourglass with an intricate pattern of "etchings"
in its walls. This picture has been composed from three
separate images taken in the light of ionized nitrogen (represented by
red), hydrogen (green), and doubly-ionized oxygen (blue).
The results are of great interest because they shed new light on the poorly
understood ejection of stellar matter which
accompanies the slow death of Sun-like stars. In previous ground-based
images, MyCn18 appears to be a pair of large outer
rings with a smaller central one, but the fine details cannot be seen.
to one theory for the formation of planetary nebulae, the hourglass shape
is produced by the expansion of a fast stellar
wind within a slowly expanding cloud which is more dense near its equator
than near its poles. What appears as a bright elliptical
ring in the center, and at first sight might be mistaken for an equatorially
dense region, is seen on closer inspection to be a potato
shaped structure with a symmetry axis dramatically different from that
of the larger hourglass. The hot star which has been thought
to eject and illuminate the nebula, and therefore expected to lie at its
center of symmetry, is clearly off center. Hence MyCn18,
as revealed by Hubble, does not fulfill some crucial theoretical expectations.
has also revealed other features in MyCn18 which are completely new and
unexpected. For example, there is a pair of
intersecting elliptical rings in the central region which appear to be
the rims of a smaller hourglass. There are the intricate patterns
of the etchings on the hourglass walls. The arc-like etchings could be
the remnants of discrete shells ejected from the star when
it was younger (e.g. as seen in the Egg Nebula), flow instabilities, or
could result from the action of a narrow beam of matter
impinging on the hourglass walls. An unseen companion star and accompanying
gravitational effects may well be necessary in
order to explain the structure of MyCn18.
Sun-like stars get old, they become cooler and redder, increasing their
sizes and energy output tremendously: they are
called red giants. Most of the carbon (the basis of life) and particulate
matter (crucial building blocks of solar systems like ours)
in the universe is manufactured and dispersed by red giant stars. When
the red giant star has ejected all of its outer layers, the
ultraviolet radiation from the exposed hot stellar core makes the surrounding
cloud of matter created during the red giant phase
glow: the object becomes a planetary nebula. A long-standing puzzle is
how planetary nebulae acquire their complex shapes
and symmetries, since red giants and the gas/dust clouds surrounding them
are mostly round. Hubble's ability to see very fine
structural details (usually blurred beyond recognition in ground-based
images) enables us to look for clues to this puzzle.